As people with light boxes will know, it's thought that blue light in particular plays a key role in setting your circadian clock, which is why it's recommended for use in the morning.

But computer (and TV screen) emissions contain a lot of blue light -- even more than daylight does. Therefore, it's been suggested that sitting in front of your computer in the evening might not be the best possible thing for your sleep.

f.lux is a nifty little piece of freeware (available in Mac and Linux versions as well as PC) that aims to solve this. You put in your geographical location, and after sunset in your area, the program "warms" the colour display of your computer screen, reducing its blueness.

You can set it to change slowly over the course of an hour, and I find it very unobtrusive (there's an easy option to disable it temporarily if you want to do anything involving precision colour work).

It may be the placebo effect, but I started using it several weeks ago and have found myself getting sleepy at an ordinary time in the evening (which is unusual for me; normally I get exhausted but not sleepy, and often have trouble sleeping) and then sleeping like a log.

Anyway, if you have sleep problems or want to improve your sleep quality, and tend to use your computer in the evening, I'd definitely recommend experimenting with this. It might also be relevant to some people with mood disorders, given how strongly SAD and bipolar in particular seem to be affected by light and circadian issues (and given the number of us crazy people who have sleep problems).